The present disclosure relates to a developing device and an image forming apparatus including the same, and, in particular, the present disclosure relates to a developing device including a first conveyance chamber and a second conveyance chamber disposed above the first conveyance chamber, each incorporating a stirring-conveyance member which stirs and conveys a developer, and an image forming apparatus including such a developing device.
In an image forming apparatus, an electrostatic latent image formed on an image carrier, such as a photosensitive member, is developed by a developing device to be visualized as a toner image. A known example of such a developing device is one that includes a developing container which holds therein a developer, first and second stirring-conveyance members which convey the developer, while stirring the developer, and a developing roller (a developer carrier) which carries thereon the developer supplied thereto from the second stirring-conveyance member. The first stirring-conveyance member conveys the developer to one side in the axial direction of the developing roller, and the second stirring-conveyance member supplies the developer to the developing roller, while conveying the developer to the other side (the side opposite from the one side).
Now, in recent years, there has been an increasing demand for the miniaturization of image forming apparatuses, and in particular, in color image forming apparatuses, in which a plurality of developing devices are disposed, there has been a demand for the miniaturization of developing devices. As an example of developing devices meeting the demand, there is known one that includes a first conveyance chamber inside which a first stirring-conveyance member is disposed and a second conveyance chamber which is disposed above the first conveyance chamber and inside which a second stirring-conveyance member is disposed. In such a developing device, it is possible to miniaturize the developing device in the horizontal direction by arranging the first conveyance chamber and the second conveyance chamber one above the other. Accordingly, it is possible to reduce space for installing the developing device, and thus, it is possible to achieve the miniaturization of image forming apparatuses.
However, in the above-described developing device, there is an area where the developer is conveyed against gravity, from the first conveyance chamber to the second conveyance chamber. With this structure, when change in flowability of the developer results from factors such as durable printing and environmental variation, the circulation balance of the developer is likely to change, and thus, uneven distribution of the developer is likely to be caused inside the developing device. When, in such a condition, toner is replenished to portions where only a small amount of developer exists, it will create portions where the concentration of the replenished toner is locally high. The replenished toner is not sufficiently mixed with a carrier included in the developer and thus is low in charge amount, and accordingly, when a portion where the concentration of the replenished toner is high is used for development, it will result in problems such as fogged images and uneven image density.
To prevent such problems, there have been proposed various methods for fully mixing the developer inside the developing device with the replenished toner. For example, there is known a developing device that has a communication path through which developer is delivered from a developing chamber to a stirring chamber by the developer falling from a downstream side of the developing chamber to an upstream side of the stirring chamber in a developer conveyance direction, and in which replenished developer replenished through a developer replenishing port above the stirring chamber falls to an area in the stirring chamber where the communication path joins the stirring chamber.